From your Exchange server try below:ping 74.6.136.244 -l 1472 -fIf above is not working, there’s a network device between you and Yahoo that’s either adding data to your packet or has a MTU smaller than yours. Keep reducing the 1472 until you can ping the target without getting any errors. Then set your server’s MTU accordingly on the NIC that’s being used to connect to Yahoo. (In my scenario it value is 1400)